1. Field of the Invention
This invention relates to dressers used to shape the surface of a grinding wheel on a grinding machine to a very high degree of accuracy. More particularly, the invention relates to a grinding wheel dresser construction that minimizes the effect of temperature variations on the accuracy of the dressing operation.
2. Description of Related Art
Precision grinding machines use a grinding wheel to accurately grind a workpiece to a desired final shape and size. As the grinding wheel is used, it slowly wears away, decreasing the accuracy of the final ground workpiece as a result of changes in both the shape of the surface and the diameter of the grinding wheel.
In order to achieve the desired precision for the final piece, the grinding wheel must be periodically reshaped and resized or "dressed" with a device known as a grinding wheel dresser. In most modern grinding machines the dresser has a dressing wheel, rotating about a dressing wheel axis, having a dressing surface coated with industrial diamond. The grinding wheel, rotating about a grinding wheel axis, is moved under the command of a numerical control unit into a known position that brings the surface of the grinding wheel into contact with the rotating dressing wheel.
As the grinding wheel and the dressing wheel contact one another (in the dressing contact region), the relative motion of the two wheels causes the diamond surface of the dressing wheel to remove a layer of grit from the surface of the grinding wheel, reshaping and truing its surface. The dressing wheel, due to its surface coating of diamond, is much harder than the grinding wheel and does not significantly change either its surface shape or its size when the grinding wheel is dressed.
Except where otherwise clear from the context, the term "surface shape" when used in connection with the grinding wheel and the dressing wheel, principally refers to the shape of the wheel at the perimeter of the wheel, not to the diameter of the wheel. Stated differently, this is the curve defined by the intersection between a plane through the axis of rotation of the wheel and the perimeter of the wheel. This is essentially the same as the curve formed by the points of contact between the grinding wheel and the dresser during the dressing operation. Thus the "shape" of the grinding wheel can be maintained by the dresser even as the diameter or size of the wheel is reduced.
At the conclusion of the dressing operation the surface shape of the grinding wheel has been brought back into the original shape, but the diameter of the grinding wheel has been slightly reduced. The software operating the numerically controlled position of the grinding wheel knows the amount of reduction because it knows the position of both the dressing wheel and the grinding wheel during the dressing operation. The reduction in the diameter of the grinding wheel is accounted for in subsequent grinding operations so that the accuracy of each workpiece being ground is maintained.
The accuracy of the subsequent grinding operation, however, depends critically upon the accuracy to which the size of the grinding wheel is known after the dressing operation. Although the surface shape of the grinding wheel is set solely by the surface shape of the dressing wheel, the size of the grinding wheel depends upon the distance between the two wheels and upon the numerical control program's knowledge about the relative location of the two wheels. Any inaccuracy in the location of the dressing contact region relative to the workpiece surface to be ground results in a mistake in the size of the grinding wheel. In turn, this inaccuracy is transferred to the workpiece during the next grinding operation.
One important source of error in sizing the grinding wheel is thermal expansion and contraction of the grinding machine to which both the grinding wheel and the dresser are attached. If the grinding machine is heated by ambient temperature increases or waste heat from motors and electronics, etc. during the grinding operation, the grinding machine will expand slightly. This expansion changes the relative positions of the dressing wheel and the grinding wheel resulting in corresponding changes in the diameter of the grinding wheel and errors in the size of the workpiece produced.
As the distance between the workpiece surface and the dressing contact region increases, the distance over which the thermal size change acts is increased. This increases the size errors in the dressing of the grinding wheel. Ideally, one would like to have the dressing contact region at the same location as the grinding contact region so that the grinding wheel was reshaped at the same location as the wheel would be used during grinding.
Unfortunately, this would result in the dresser being mounted where the workpiece must be located. The conventional position for the dresser has been to mount the dresser on the opposite side of the grinding wheel from the workpiece so that it is clear of the working area and yet still has access to the grinding wheel surface.
In this configuration the surface of the grinding wheel is dressed on the back side of the grinding wheel, positioning the dressing contact region at a point 180 degrees from the front side of the wheel where the grinding contact region between the wheel and the workpiece is located. However, this placement results in locating the dressing contact region much farther from the axis of rotation of the workpiece than the grinding contact region. The disparity in this distance magnifies the thermal expansion errors when dressing the grinding wheel.
A principal object of the present invention is to avoid thermal expansion errors by locating the dressing wheel and the dressing contact region such that thermal expansion errors are eliminated or substantially reduced.